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Operando Raman to Enhance the Methanol-to-DME Conversion Over Non-thermally-pre-treated Keggin Heteropolyacids Josefine Schnee, and Eric M. Gaigneaux J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11248 • Publication Date (Web): 19 Dec 2016 Downloaded from http://pubs.acs.org on January 3, 2017
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The Journal of Physical Chemistry
Operando Raman to Enhance the Methanol-to-DME Conversion Over Nonthermally-pre-treated Keggin Heteropolyacids Josefine Schnee, Eric M. Gaigneaux* Institute of Condensed Matter and Nanosciences (IMCN) – MOlecules Solids and reactiviTy (MOST) – Université catholique de Louvain (UCL). Place Louis Pasteur 1, box L4.01.09 1348 Louvain-la-Neuve, Belgium.
ABSTRACT Heteropolyacids (HPAs) are metal-oxygen clusters which are nowadays widely used in acid catalysis. Indeed, as they possess a very strong Brönsted acidity, approaching the super-acid region, they generally allow performing reactions at lower temperatures than conventional catalysts. In the present paper, we use in situ/operando Raman spectroscopy to optimize the catalytic performance of H3PW12O40 - the strongest Keggin-type HPA - in the low temperature (150°C) gas phase dehydration of methanol to dimethylether (DME), which is one of the most promising renewable fuels for the future. Precisely, we demonstrate that the ability of methanol to displace the HPA’s crystallisation water located in-between the Keggin units – and thus to reach the acidic protons – decreases with increasing temperature. Actually, we show that one and the same flow of methanol scarcely displaces the crystallisation water at reaction temperature 150 °C; whereas, at a temperature as low as 25 °C, it succeeds to completely dehydrate the HPA. By exploiting this, namely by pre-exposing the initially hydrated HPA to methanol at 25 °C right before heating it to reaction temperature 150 °C, the conversion increases dramatically, precisely by a factor 3. As reflected by the spectra, the flow of methanol at 25 °C does not only dehydrate but also re-crystallizes the HPA (so rendering its structure more rigid and the diffusion in-between the Keggin units later at 150 °C more difficult). As a consequence, the conversion obtained at 150 °C is not as high as after a thermal dehydration under inert atmosphere. However, to be complete, the latter requires a temperature as high as 320 °C which is not tolerated by all HPA-based catalysts. In other words, the present work shows how to pre-treat an HPA-based catalyst in order to maximize its catalytic performance in the methanol-to-DME reaction, depending on its resistance or not to a harsh thermal pre-treatment.
1. INTRODUCTION Heteropolyacids (HPAs) are metal-oxygen clusters which are widely used in acid catalysis. Indeed, thanks to their very high Brönsted acidity, approaching the super-acid region, they generally allow efficient operation under milder conditions than conventional acid catalysts (e.g. zeolites, γ-Al2O3).1,2 Among a large diversity of structures, Keggin HPAs are the most studied. Indeed, they are the most easily available ones.1,3 The Keggin unit is composed of an heteropolyanion having the formula [XM12O40]n- – where X is the heteroatom (often P5+, Si4+) and M is the addenda atom (typically Mo6+, W6+) – and being stabilized by n acidic protons.3 More precisely, the heteropolyanion is made of a central XO4 tetrahedron, surrounded by 12 MO6 octahedra. It contains 3 types of oxygen atoms: central (Oa), bridging (either corner-sharing (Ob) or edge-sharing (Oc)), and terminal (Ot) ones.4 A reaction that attracts more and more attention nowadays, and for which Keggin HPAs are excellent catalyst candidates, is the gas phase dehydration of methanol to dimethylether (DME). DME is actually considered as the “fuel for the 21st century”.5 It can be used in diesel trucks, for power generation, in fuel cells and even to replace LPG as cooking gas.5,6 Its combustion leads to almost no emission of particulates; it is non-toxic, non-corrosive and biodegradable in air.5,7 In other 1 ACS Paragon Plus Environment
The Journal of Physical Chemistry
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words, it has a very low climate impact. Keggin HPAs have been shown to exhibit high catalytic activities in the methanol-to-DME process at temperatures as low as 140 °C, reaching conversions for which conventional catalysts require much higher temperatures (typically 300 °C).8 Nevertheless, there is still a great challenge to address in order to optimize their performance. Indeed, thanks to their so-called “pseudo-liquid behavior”, HPAs are actually able to absorb polar reactants such as alcohols and ethers into their bulk.6,9,10 As a direct consequence, the latter molecules may potentially react both at the surface and within the bulk of HPA crystals. However, within the bulk, the Keggin units (primary structure) are actually coordinated with crystallisation water molecules (secondary structure). Precisely, the most common form contains six crystallisation water molecules per Keggin unit, leading to H5O2+ bridges along the faces of a body-centered cubic structure with the Keggin anions at the lattice points.1 This means that, to get protonated and subsequently converted into DME on all the HPA’s acidic protons, methanol needs to displace the whole crystallisation water within all the H5O2+ bridges.8 This is however not straightforward8; and it can often not be circumvented by simply applying a thermal pre-treatment to evacuate the crystallisation water. Indeed, to get a completely anhydrous HPA, a pre-treatment up to 320 °C is required2; and, depending on the properties of a given HPA-based catalyst (e.g. nature of the support, desired size of HPA crystals), such a pre-treatment is not always tolerable. In the present paper, we show a way to enhance the performance of Keggin HPAs in the methanol-to-DME reaction that does not require to evacuate the crystallisation water by performing a thermal pre-treatment. On the contrary, it is based on a pre-treatment at room temperature (25 °C). Precisely, we demonstrate via operando Raman spectroscopy that the ability of methanol to displace a Keggin HPA’s crystallisation water actually depends on temperature; and that, by exploiting this properly, the yield of DME can be enhanced dramatically. With the aim of getting the highest possible conversion, we have used the most acidic Keggin HPA, namely phosphotungstic acid (H3PW12O40). The operando methodology, finding many applications nowadays11,12,13,14, is defined as a “combined spectroscopic measurement and simultaneous online measurement of catalytic activity/selectivity values during catalytic reaction studies”.15 The term operando means “working/operating” in Latin. So, the catalyst is monitored at work in real time, instead of before and after the reaction as it is classically done. In particular, Raman spectroscopy allows operando monitoring without significant interference from the gas phase.15
2. EXPERIMENTAL SECTION 2.1. Chemicals H3PW12O40 (hereafter HPW12) has been purchased from Sigma-Aldrich in the form of H3PW12O40.xH2O (reagent grade). The identity of the sample was confirmed by X-ray diffraction (not shown). The powder has been placed overnight under vacuum (
